Method for producing a sintered silicon nitride base ceramic and said ceramic

ABSTRACT

A silicon nitride based ceramic is formed of silicon nitride and at least two metal oxides of such a type that when the metal oxides are heated separately they form a spinel. Combination of said metal oxides and said silicon nitride as fine powders and sintering same at a specified temperature for a specified period of time results in a silicon nitride based ceramic having improved mechanical and chemical properties.

BACKGROUND OF THE INVENTION

Recently, ceramics composed principally of silicon nitride have foundwide use because of the fact that they display superior heat resistance,superior thermal shock resistance, high strength and high corrosionresistance against molten non-ferrous metals and against oxygen at hightemperature. In general, it is difficult to prepare silicon nitrideceramics by sintering so that it is difficult to obtain a siliconnitride ceramic having low porosity and high strength by any methodother than hot-pressing. However, the hot-pressing method is applicableonly to those cases in which the product is of a relatively simpleprofile, so that a method for forming a sintered silicon nitrate ofcomplex profile having a low porosity and high strength has been soughtin this art.

A technique other than hot-pressing which has been used is to form acompact of silicon powder and to heat the compact gradually in anatmosphere of nitrogen or gaseous ammonia so that the silicon isnitrided and sintered at the same time. However, the porosity of theproduct is generally at least 20 percent so that the strength of theproduct is low.

SUMMARY OF THE INVENTION

A powder mixture containing 60 to 92 mol percent of silicon nitride withthe remainder being metal oxide is prepared. The metal oxide consists ofat least one member selected from a first group consisting of MgO, ZnOand NiO, and at least one member of a second group consisting of Al₂ O₃,Cr₂ O₃, Y₂ O₃, TiO₂ and SnO₂. The mol ratio of member or members of thefirst group to member or members of the second group lies between 1:9and 9:1. In a first embodiment of the process, the mixed metals areheated to betweeen 1600° and 1800°C for 2 to 3 hours. In a secondembodiment of the invention the members of the two groups of metaloxides are first heated to a temperature high enough and for a periodlong enough to form a spinel, then cooled and reground, after which themetal oxides in the form of a spinel are mixed with the silicon nitrideand sintered as above. Sintering is carried out in an inert atmospheresuch as nitrogen or argon.

An object of the present invention is an improved silicon nitride basedceramic having improved strength and corrosion resistance.

Another object of the present invention is an improved silicon nitridebased ceramic containing metal oxides, said ceramic having high strengthand high corrosion resistance.

A further object of the present invention is an improved silicon nitridebased ceramic including metal oxides from at least two groups where theceramic has high strength and high corrosion resistance.

Yet another object of the present invention is an improved siliconnitride based ceramic containing metal oxides in a proportion such thatthe metal oxides themselves may be formed into a spinel when heated to asufficiently high temperature for a sufficiently long period of time.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and thecomposition possessing the features, properties, and the relation ofconstituents, which are exemplified in the following detaileddisclosure, and the scope of the invention will be indicated in theclaims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Sintered articles based on silicon nitride and having the properties oflow porosity, high strength and high corrosion resistance are obtainedby sintering said silicon nitride in combination with metal oxides whichare such that when heated alone to a suitable sintering temperaturespinels are formed. Spinels are crystals of the cubic system and havethe general formula RO.R'₂ O₃ where R and R' are metal elements and thepure spinel consists of one mol of divalent oxide and one mol oftrivalent oxide. Further, it is known that spinels can be formed betweena divalent metal oxide and a tetravalent metal oxide such as that oftitanium or tin or the like. Where the spinel is made from a divalentand a tetravalent metal oxide, the usual ratio is 2 mols of a divalentoxide to 1 mol of the tetravalent metal oxide. Tests have shown thatsintered articles of low porosity and high strength are not alwaysobtained when silicon nitride powder is combined with oxides in such aratio as to form a spinel. However, the desired properties can beobtained with a restricted group of oxides where the ratio of the oxidesto each other and the ratio of the oxides to the silicon nitride liesbetween specific limits. The metal oxides which have been found suitablefor forming silicon nitride based ceramics of high strength and highcorrosion resistance are MgO, ZnO and NiO as the divalent metal oxides,Al₂ O₃, Cr₂ O₃ and Y₂ O₃ as the trivalent metal oxides and TiO₂ and SnO₂as the tetravalent metal oxides.

In accordance with the present invention, the divalent metal oxides areregarded as forming one group and the trivalent and tetravalent metaloxides are regarded as a second group. In the compositions in accordancewith the present invention at least one member is selected from thefirst group and at least one member is selected from the second group tobe sintered with the Si₃ N₄.

In addition to the above metal oxides, a small quantity of CaO or CoOmay be included in the sintered silicon nitride based ceramic withoutlowering the strength thereof. However, tests have shown that othermetal oxides may lower the strength and corrosion resistance of thesilicon nitride based ceramic produced. Nevertheless, other advantagesmay be obtained such as lowering the sintering temperature. The additionof silicon dioxide produces this effect.

As aforenoted, the pure spinel structure is formed between one mol ofthe divalent metal oxide and one mol of the trivalent metal oxide, orbetween two mols of the divalent metal oxide and one mol of thetetravalent metal oxide. However, in accordance with the presentinvention, 10 to 90 mol percent of the divalent metal oxides are usedwith 90 to 10 mol percent of the trivalent or tetravalent metal oxides,said metal oxide being selected from the two groups presented above. Theproduce formed when the members of the two groups of oxides are not inthe true spinel ratio is not exactly a spinel but rather a solidsolution having a spinel-like structure. Such spinel-like materials incombination with silicon nitride give a sintered product which has theimproved properties desired to an extent which is quite comparable withwhat is achieved by using said materials in the ratio necessary forgiving a pure spinel.

When the silicon nitride content is less than 60 mol percent (i.e., theoxide content is more than 40 mol percent), the strength of the siliconnitride ceramic decreases and owing to the metal oxides havingrelatively low strength, the strength of the article is seriouslylowered. Also, when the silicon nitride content is more than 92 molpercent (i.e., the spinel content is less than 8 percent), it becomesdifficult to sinter the material, and a product of low porosity cannotbe easily obtained, and therefore its strength is remarkably lowered.Therefore, the silicon nitride content in the mixed material powder islimited from 60 to 92 molar percent.

The reason why the superior sintered article of low porosity and highstrength can be obtained by combining the silicon nitride powder withsaid metal oxides to form spinels has not yet completely been understoodin detail. However, the following is believed to be the mechanism. Themagnesium oxide or aluminum oxide is believed to dissolve in the siliconnitride in the sense that metal atoms from the oxides are substitutedfor the silicon in the nitride and the magnesium and aluminum ions takeon tetrahedral coordination. But since positive ions of magnesium oxideor aluminum oxide alone form structures of hexahedral coordination andthe structure must change over to tetrahedral coordination for the solidto be dissolved in silicon nitride, consequently, a high activationenergy is needed. In contrast, most of the positive ions of spinel forma tetrahedral coordinated structure which is similar to the case of asolid dissolved in silicon nitride so that the activation energy for thesolution process is low and the sintering operation progresses easily.Thus it is considered the reason for superior properties of siliconnitride in combination with oxides which form spinels, or spinel-likestructures is accounted for.

With respect to the process of the present invention as described above,it is believed that the reaction which occurs first during the sinteringoperation is the conversion of the metal oxides to spinel-likestructures and then the spinels dissolve in the silicon nitride topromote the sintering operation of the silicon nitride.

While the process of the present invention can be carried out by mixingthe metal oxides together with the silicon nitride, or compounds beingin the form of fine powders, and then sintering, a preferred method isto combine one or more members of the first group of metal oxides withone or more members of the second group of metal oxides in finelypowdered form and to sinter the metal oxides alone. The sintered metaloxides which have now formed a spinel-like structure are then finelyground and mixed with the silicon nitride and formed into a compact tobe treated as above. Using this method, the product obtained aftersintering has lower porosity and higher strength than the productobtained by the first method. It is believed that the reason is that themetal oxides react more effectively to form the desired spinel-likestructure so that the conversion is more complete and in the sinteringoperation, the dissolution process by which the spinels are dissolved inthe silicon nitride to aid the sintering operation progresses moreeffectively. Following are examples describing the method of preparationof the ceramics in accordance with the present invention.

EMBODIMENT 1

60 to 92 mol percent of silicon nitride powder having a maximum size ofless than 300 mesh and having a purity of 98% is mixed with 4 to 36 molpercent of magnesium oxide of the same mesh and high purity and 4 to 36mol percent of alumina of similar quality. The mixture is compacted at apressure of 500 kg/cm² and sintered for 2 to 3 hours at a temperature of1600° to 1800°C in an inert atmosphere, preferably of argon or nitrogen.By this means, samples of 40mm × 20mm × 6mm are produced and the variousproperties such as specific gravity, porosity, bending strength,oxidation resistance and thermal-expansion coefficient may be measured.Specific sample compositions with their properties are shown in Table 1.Also, for comparison, the properties of compositions with siliconnitride obtained with aluminum oxide alone and with magnesium oxidealone are given for comparison in the same Table.

The methods of measuring the properties shown in the various Tables wereas follows. The porosity was calculated from the measured specificgravity assuming that the specific gravity of sintered silicon nitrideis 3.19 when its porosity is 0 percent. The bending strength wasmeasured by means of the 3-point supporting method, the span of theouter points being 30mm. For the oxidation resistance test, theincrement in weight was measured after 48 hours at 1200°C in air, andthe result was shown as the increase in weight per unit area, measuredin mg/cm². For the thermal-expansion coefficient, the article was heatedfrom 20°C to 400°C and the average thermal-expansion coefficient withinthe range was measured. Now noting the values given in Table 1, bysintering mixtures of powder containing 60 to 92 mol percent of siliconnitride together with mixtures of metal oxides in accordance with thepresent invention where the MgO content ranges from

                                      Table 1                                     __________________________________________________________________________                                                            Thermal-                                                                      expansion                                                Specific  Bending                                                                            Oxidation                                                                           coefficient                  Sample                                                                            Quantity (mol %)                                                                            Sintering gravity   strength                                                                           resistance                                                                          (20-400°C)                                                             8                            number                                                                            Si.sub.3 N.sub.4                                                                   Al.sub.2 O.sub.3                                                                   MgO Temperature(°C)                                                                  (g/cm.sup.3)                                                                       Porosity                                                                           (kg/mm.sup.2)                                                                      (mg/cm.sup.2)                                                                       (×10.sup..su                                                            p.-6 /°C)      __________________________________________________________________________    Samples of                                                                           1   92   4     4  1750      3.01  7   31.0 2.8   2.5                   the present                                                                          2   70   4    26  1750      2.70 15   24.0 5.2   2.4                   invention                                                                            3   60   4    36  1750      2.75 14   24.8 6.3   2.4                          4   90   5     5  1750      3.05  4   40.0 1.5   2.4                          5   79   6    15  1780      3.03  5   38.5 1.8   2.4                          6   79   6    15  1600      2.70 15   23.0 10.6  2.4                          7   65   15   20  1750      3.00  6   32.5 2.0   2.4                          8   76   20    4  1750      2.86 10   29.2 3.6   2.4                          9   60   36    4  1750      2.90  9   30.6 2.8   2.4                   Samples for                                                                          101 85   15    0  1780      2.53 20   16.0 10.0  2.4                   comparison                                                                           102 75   0    25  1780      2.40 25   15.5 15.0  2.4                   __________________________________________________________________________     90 mol percent (sample 3) to 10 mol percent (sample 9), it is apparent     from Table 1 that the porosity is from 4 to 15 percent, the bending     strength is from 23 to 40 kg/mm.sup.2 and the oxidation resistance is from     1.8 to 10.6 mg/cm.sup.2. These values are definitely superior to those of     the comparison samples which showed a porosity of 20 to 25 percent,     bending strength of 15 to 16 kg/mm.sup.2 and an oxidation resistance of 10     to 15 mg/cm.sup.2. The superiority of the product resulting from     compositions in accordance with the present invention is particularly     evident in the silicon nitride ceramic of sample 4 where the porosity is 4     percent, the bending strength is 40.0 kg/mm.sup.2 and the oxidation     resistance is 1.5 mg/cm.sup.2, these values being nearly equal to those of     sintered articles of silicon nitride formed by the hot-press method.

With respect to the sintering temperature, at temperatures lower than1600°C it is difficult to obtain articles of low porosity whereas whenit is higher than 1800°C a decrease in weight was observed indicatingeither that decomposition or sublimation was occurring. With respect tothe sintering time, the reactions are not complete if the period is lessthan 2 hours and where the period is longer than 3 hours, the growth ofcrystal particles of silicon nitride was observed. So far as the inertgas is concerned, no difference was noted between the products made inargon or in nitrogen. Summing up, the preferable sintering conditionsare a temperature of 1600°C to 1800°C, a sintering time from 2 hours to3 hours, and an inert gas atmosphere.

EMBODIMENT 2

Silicon nitride, aluminum oxide and magnesium oxide powders similar tothose used in the examples of embodiment 1 were employed. According tothis embodiment, initially, magnesium oxide and aluminum oxide weremixed in several mol ratios and were heated for 3 to 10 hours at atemperature of 1600°C to 1800°C in order to form spinels. After thisreaction was complete, the spinels were cooled and pulverized to aparticle size finer than 300 mesh, this powder was mixed with siliconnitride powder of the same mesh, after which the powder mixture wastreated as in Embodiment 1. The properties were measured. Examples aregiven in Table 2.

                                      Table 2                                     __________________________________________________________________________                                                            thermal-                                                                      expansion                                         sintering                                                                            specific  bending                                                                            oxidation                                                                           coefficient                  sample                                                                            quantity (molar %)                                                                             temperature                                                                          gravity                                                                            porosity                                                                           strength                                                                           resistance                                                                          (20-400°C)                                                             7                            number                                                                            Si.sub.3 N.sub.4                                                                   Al.sub.2 O.sub.3                                                                    MgO   (°C)                                                                          (g/cm.sup.3)                                                                       (%)  (kg/mm.sup.2)                                                                      (mg/cm.sup.2)                                                                       (×10.sup..su                                                            p.-6 /°C)      __________________________________________________________________________    Samples of                                                                           10  60   4(10) 36(90)                                                                              1780   3.06 4.0  41.8 0.9   2.38                  the present                                                                          11  90   5(50)  5(50)                                                                              1780   3.13 1.8  65.0 0.5   2.40                  invention                                                                            12  79   6(29) 15(71)                                                                              1780   3.13 1.8  58.0 0.6   2.38                         13  79   6(29) 15(71)                                                                              1600   3.00 2.6  32.0 1.2   2.38                         14  60   30(75)                                                                              10(25)                                                                              1780   3.05 4.5  40.6 0.9   2.38                  __________________________________________________________________________     NOTE:                                                                         Figures in parentheses are based on oxide content only.                  

It is apparent from the Table 2 that the porosity is 1.8 to 4.5 percent,the bending strength is 32 to 65 Kg/mm² and the oxidation resistance is0.6 to 1.2mg/cm² with the sintered substances produced by this method.These values are much better than those of the sintered articles ofsilicon nitride produced by the method of the embodiment 1 shown in theTable 1. Especially, with the sintered article of silicon nitride of thesample number 11, the porosity is 1.8 percent, the bending strength is65.0 kg/mm² and the oxidation resistance is 0.5mg/cm² ; the values ofthe properties are nearly equal to those of the sintered article ofsilicon nitride obtained by means of the hot-press method.

A crucible of 50mm height, 40mm inner diameter and 3mm thickness wasformed of the sintered silicon nitride of the sample number 11 by theslip casting method, and it was sintered for 3 hours at 1750°C in anatmosphere of nitrogen gas. The crucible thus obtained was used formelting aluminum. Even when the aluminum was heated to 1100°C, thecrucible was not attacked, the surface of the crucible was not oxidized,and also aluminum did not permeate into the crucible. In contrast, whena crucible of the sintered silicon nitride formed by the conventionalreactive sintering method was tested at 900°C with aluminum, the surfacewas first oxidized and the crucible was eventually destroyed.

EMBODIMENT 3

Mixed material of divalent metal oxides (MgO, ZnO, NiO) and trivalentmetal oxides (Cr₂ O₃, Al₃ O₃, Y₂ O₃) having the same mol quantity aseach other, and the mixed powder of divalent metal oxide (MgO) andtetravalent metal oxides (TiO₂, SnO₂) in a mol ratio of 2 to 1 (eachpowder is finer than 300 mesh) were respectively made by the same methodas that of the second embodiment. They were heated for 3 to 10 hours attemperatures of 1300° to 1600°C to form the spinels (MgCr₂ O₄, ZnAl₃ O₄,NiAl₂ O₄, MgY₂ O₄, 2 MgO.TiO₂, 2MgO.SnO) respectively. After thisprocess, the spinels were pulverized to powder (finer than 300 mesh),and the powder was mixed with silicon nitride powder to make the mixedmaterial, and the sintered articles of silicon nitride were produced bythe same method as that of the first embodiment, and then the propertiesof the articles were measured. The compositions and results are shown inTable 3.

                                      Table 3                                     __________________________________________________________________________                                                            thermal-                         Quantity (Mol %)                             expansion                             Spinels     Sintering                                                                            Specific  Bending                                                                            Oxidation                                                                           coefficient                  Sample         R'.sub.2 O.sub.3                                                                    Temperature                                                                          Gravity                                                                            Porosity                                                                           Strength                                                                           resistance                                                                          (20-400°C)                                                             N                            number                                                                            Si.sub.3 N.sub.4                                                                   RO    R"O.sub.2                                                                           (°C)                                                                          (g/cm.sup.3)                                                                       (%)  (kg/mm.sup.2)                                                                      (mg/cm.sup.2)                                                                       (×10.sup..su                                                            p.-6 /°C)      __________________________________________________________________________    Samples of                                                                           15  90   MgO 5 Cr.sub.2 O.sub.3                                                                  5 1780   3.14 2    50.2 0.87  2.8                   the present                                                                          16  90   ZnO 5 Al.sub.2 O.sub.3                                                                  5 1780   3.02 7    40.1 2.9   2.6                   invention                                                                            17  90   NiO 5 Al.sub.2 O.sub.3                                                                  5 1780   3.08 4    42.3 2.8   2.6                          18  90   MgO 5 Y.sub.2 O.sub.3                                                                   5 1780   3.17 0.5  45.0 0.8   2.6                          19  90   MgO 6.7                                                                             TiO.sub.2                                                                         3.3                                                                             1780   3.09 3    46.7 1.6   2.6                          20  90   MgO 6.7                                                                             SnO.sub.2                                                                         3.3                                                                             1780   2.97 8    36.5 5.2   2.6                   __________________________________________________________________________

It is apparent that the sintered article obtained by sintering the mixedpowder of silicon nitride and spinels formed of magnesium oxide, zincoxide and nickel oxide as divalent metal oxides, aluminum oxide,chromium oxide and yttrium oxide as trivalent metal oxides and titaniumoxide and tin oxide as tetravalent metal oxides show superior strength(the bending strength being 36.5 to 50.2 kg/mm²), low porosity (0.5 to 8percent) and superior oxidation resistance (0.87 to 2.9 mg/cm²).

As mentioned above, according to the present invention, 60 to 92 molpercent silicon nitride and 8 to 40 mol percent metal oxides arecombined and mixed together with one another to comprise 100 mol percentof the final product as the material powder. Alternatively, the metaloxides are previously sintered to form spinels, and the spinels areminutely pulverized to powder. The powder is mixed with the siliconnitride powder to make the mixed material powder. Either of the twotypes of material powder are compacted by conventional methods, andsintered in non-oxidizing atmosphere to produce a sintered article ofsilicon nitride. Also, said metal oxides are composed of one kind ormore than one kind of 10 to 90 mol percent magnesium oxide, zinc oxideand nickel oxide, and of one kind or more than one kind of 10 to 90 molpercent of aluminum oxide, chromium oxide, titanium oxide, tin oxide andyttrium oxide. By this method, sintered articles of silicon nitrideceramic having a complicated profile of superior properties, especiallyof high strength, can be easily produced at low cost. Further, a simplemethod which produces an article of superior quality and which has beendesired is achieved.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in carrying out the above process andin the composition set forth without departing from the spirit and scopeof the invention, it is intended that all matter contained in the abovedescription shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention, which, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A process of preparation of a silicon nitridebased ceramic comprising the steps of combining 60 to 92 mol percent offinely powdered Si₃ N₄ with at least one member of a first groupconsisting of MgO, ZnO and NiO and at least one member of a second groupconsisting of Al₂ O₃, Cr₂ O₃, Y₂ O₃, TiO₂ and SnO₂, said Si₃ N₄ powderand said members being in powdered form finer than 300 mesh, the molratio of said first group member or members to said second group memberor members ranging from 1:9 to 9:1 and the combined total quantity ofsaid members lying between 8 and 40 mol percent of the combinationincluding said Si₃ N₄, mixing said powdered materials, forming saidmixed powders into a green compact and sintering said green compact at atemperature between 1600°C and 1800°C in an inert gas in the absence ofapplied pressure, the resultant composition consisting essentially ofSi₃ N₄ and members of said first and second groups.
 2. The process asdefined in claim 1 wherein said compact is formed at a pressure of atleast about 500 Kg/cm².